Why are the French thinner than Americans? In the land of six-cup-holder SUVs and super sized fries, the news from Psychology Professor Paul Rozin is that it’s the portion size, stupid.

“We saw what every tourist knows,” said Rozin, “but we measured it. Not only are portions smaller in France, people spend more time eating.” His findings were published this month in Psychological Science, the journal of the American Psychological Society.

Rozin recruited undergraduate and graduate students at Penn, along with some colleagues from France to march into a variety of comparable restaurants, portable digital scale in hand and measure the meals item by item. The results: “On average, American portions are 25 percent larger,” Rozin reports.

The news from the fast-food chains like McDonald’s and Pizza Hut, with outlets on both continents, were particularly interesting. Hamburgers and a six-piece portion of chicken nuggets (both shipped to local stores) were identical. On other items where serving sizes were locally determined—fries and soda—sizes were larger in Philadelphia.

“Probably the single most important determinant of meal intake is how much is served,” says Rozin. “[In this country], the problem is compounded by the mega-sizes that nobody eats. Studies have shown that people serve themselves bigger portions out of the larger sizes. If you ask someone to pour a Coke out of a two-liter bottle they will pour more.”

Rozin and his bunch also compared guidebooks looking at mentions of portion size, cookbooks, and supermarkets.

Choice is another important factor. “One of the things that strikes you when you eat in a French restaurant is you don’t get that much choice. At the Amazon Juice place at Penn, I calculated that there are over 6,000 possible combinations that you can order there,” reports Rozin. “It turns out that there is a fair amount of data that people eat more when there are more choices.”

Rozin has been at work for many years studying American and French attitudes towards food and health. “In the U.S. there is a tremendous ambivalence about what I take to be one of the greatest pleasures in life. For the French, food is a sensory experience.”

In a six-county study on attitudes about food sponsored by the French Dairy Council, Rozin and his associates found that Switzerland, France, Italy and Germany are alike, the U.S. is quite different and Britain is in-between. “We asked people what was the best metaphor for the relationship between food and the body. People in the U.S. and U.K. favored the factory or the car. France favored the tree. We have this mechanical, physiological view.”

Rozin doesn’t pretend to have solved the so-called “French paradox”—why the French on a diet rich in fats are thinner and live longer than Americans. “I am saying that portion size has something to do with lower weight and probably greater health advantages. I don’t think that it is the answer.”